Power distribution systems

ABSTRACT

A power distribution system that can be used in naval ships and submarines includes a first power generation system including at least one power source for supplying power to a first service distribution system that includes at least one dc distribution busbar for carrying a distribution voltage and a distribution current and at least one switchboard that includes protective switchgear with contacts. A zonal power distribution sub-system includes a zonal power supply unit for supplying power to at least one electrical load and a zonal energy store connected to the at least one switchboard of the first service distribution system for supplying power to the zonal power supply unit. The at least one power source is regulated according to a power source foldback and stabilizing characteristic and a power source starting characteristic. The at least one electrical load is regulated according to a load shedding and stabilizing characteristic.

FIELD OF THE INVENTION

The present invention relates to a power distribution system, and inparticular to a “smart” power distribution system that can be used formarine power and propulsion systems. The power distribution systemenables a marine power and propulsion system to achieve maximum powerdensity, efficiency and stability and facilitates future technologyinsertions by means of a modular approach and standard interfaces.

BACKGROUND OF THE INVENTION

In conventional marine power and propulsion systems that employ fullelectric propulsion (FEP), fault current magnitude-time discriminationis used to enable protective switchgear to interrupt over-current faultsin particular sub-circuits while causing the minimum practicaldisruption to all other sub-circuits. Such FEP systems are said toemploy the “power station principle” where the aim is to adapt the powergeneration capacity that is on-line at any particular time to the totalload that is being drawn at that time. This has the effect of maximizingfuel efficiency. The configuration of such FEP systems is normallyautomated to some degree by a power management system with the authorityto shed load and start generators in a prioritized manner. Alternatingcurrent is distributed through the FEP systems at medium voltage (MV) tomaintain compatibility with land-based systems.

An example of a conventional FEP system is shown in FIG. 1. A series ofturbines T and diesel engines D are used to power individual generatorsG. These supply ac power to the FEP system through a medium voltage (MV)ac busbar system that is equipped with protective switchgear. Theprotective switchgear comprise circuit breakers and associated controlsand are represented in FIG. 1 by the x symbol. Power converters PC areused to interface the MV ac busbar system to an electric propulsionmotor PM that drives a propeller. Filters F are also connected to the MVac busbar system. The MV ac busbar system is divided into a first MV acbusbar and a second ac MV busbar that are interconnected by protectiveswitchgear. A first low voltage (LV) ac busbar is connected to the firstMV ac busbar through a first transformer. A second LV ac busbar isconnected to the second MV ac busbar through a second transformer. Thefirst and second LV ac busbars are interconnected by protectiveswitchgear. A series of unspecified large and minor loads can beconnected to the first and second LV ac busbars, respectively. It willbe clear from FIG. 1 that the minor loads are connected to the first andsecond LV ac busbars through first and second minor LV ac busbars.

Six magnitude-time discrimination levels of the FEP system are shownalong the right hand side of FIG. 1. Protective switchgear isrepresented by the x symbol in each of the discrimination levels. Forexample, in discrimination level 6 protective switchgear is locatedbetween the MV ac busbar and each of the generators G. In discriminationlevel 5 protective switchgear is located between the MV ac busbar andeach of the filters F and between the MV ac busbar and each of the powerconverters PC. Protective switchgear is located between the MV ac busbarand each of the transformers that are used to connect the first andsecond MV ac busbars to the first and second LV ac busbars,respectively. In discrimination level 4 protective switchgear is locatedbetween each of the transformers and the respective LV ac busbars. Indiscrimination level 3 protective switchgear is located between thefirst and second LV ac busbars and each of the large loads and betweeneach of the respective feeds to the minor LV ac busbars. Indiscrimination level 2, further protective switchgear is located betweenfirst and second LV ac busbars and the associated parts of the minor LVac busbars. In discrimination level 1 protective switchgear is locatedbetween the minor LV ac busbars and each of the minor loads.

A short circuit in any particular discrimination level of the FEP systemmust trip the associated protective switchgear in that level but mustnot cause any other protective switchgear to trip. Protective faultcurrent levels are determined entirely by supply impedance and theprotective switchgear is only able to interrupt the fault current (i.e.,the current flowing in the FEP system during a fault) well after thepeak fault current has passed. The fault current is therefore normallyonly interrupted at, or very shortly after, line current reversals.

The conventional FEP system shown in FIG. 1 has the following technicaldisadvantages.

The magnitude of the fault current is influenced by the number and typeof generators G that are on-line on a particular point of commoncoupling; the lower the combined generator impedance the greater thefault current. Wide variations in prospective fault current occur andprotection equipment setting may have to be continuously adjustable toguarantee fault discrimination.

The magnitude of the fault current is increased as distribution voltage(i.e., the voltage carried by the various ac busbars in the FEP system)is reduced. As the total installed power rating is increased and/ordistribution voltage is reduced, the resulting fault current may exceedthe capability of the available protective switchgear. Medium voltagepower distribution systems may have to resort to the use of loadstep-down transformers and specialized insulation systems in order toallow a sufficiently high distribution voltage to be used to overcomeprotective switchgear limitations.

The characteristics of the generators G may vary widely in terms of timedependency and peak magnitudes of ac and dc components to aid loadsharing. (Automatic Voltage Regulators (AVRs) are designed to aid loadsharing.) Moreover, these characteristics are greatly influenced by thetype of prime movers (diesel engine D or turbine T, for example) that iscoupled to the generator and their resultant coupled governed andregulated responses may be subject to significant disparities. When agroup of generators G is connected to a point of common coupling thendisparities often become problematic, particularly during the switchingof passive circuits such as filters and transformers and during loadtransients.

The FEP system is often split into multiple points of common couplingthat are often referred to as “islands”. All islands may be connectedtogether in parallel to give a single island arrangement (e.g., forsingle engine running) or may be separated to provide redundancy andgraceful degradation of capability following equipment failures.Synchronization and load transfer between individual islands iscomplicated, particularly when they have different degrees of harmonicpollution and when the disparities mentioned above are present.Propulsive power is normally drawn from the islands in a PropulsionDistribution System (PDS) and other loads can be fed by islands in aShip Service Distribution System (SSDS) whose power is usually derivedfrom the PDS. Protective discrimination and quality of power supply areusually related by common hierarchy that extends from the largestgenerator G down to the smallest electrical load. Means must be providedto decouple the relatively sensitive SSDS from the potentially harmfuleffects of the relative robust power and propulsion equipment in thePDS. Critical electrical loads may require local high integrity powersupplies of their own with dedicated power conversion and energy storageequipment in order to attain the required degree of decoupling from thePDS. These local power supplied are often referred to as Zonal PowerSupply Units (ZPSU) and their energy stores are often referred to asZonal Energy Stores (ZES).

Since the FEP system is an ac system a number of variables can affectits design. These include inter alia voltage, frequency, phase angle,power factor, point in cycle switching events, phase imbalance, integerand non-integer harmonic distortion. Because it is a complex ac systemit is recognized that it is very difficult to damp the inevitableresonant modes between stray and intentional impedances that affect sucha power distribution system. Once an ac distribution frequency (i.e.,the frequency of the ac current carried by the various ac busbars in theFEP system) has been chosen then this will greatly influence thegenerator topology and ultimately places limits on the shaft speed ofthe prime mover. In many cases, this will adversely affect the size andperformance of the generator and the prime mover.

While most conventional FEP systems distribute ac current at mediumvoltage (MVAC), it is also known to distribute dc current at low voltage(LVDC). Although these LVDC systems derive their dc current from MVACcurrent supplies via current limited power electronics, they rely on dccircuit breakers (DCCB) to interrupt significant fault currents.

For example, an SSDS may use phase-controlled transformer rectifiers toderive a LVDC distribution voltage from a conventional MVAC distributionsystem. Parallel redundant feeders distribute the LVDC distributionvoltage through switchboards that include fault current-rated DCCBs.Each ZPSU is fed from a redundant pair of these switchboards viainterposing regulated power electronics and anti-backfeed diodes.

Another SSDS may use transformer-isolated back-to-back pulse widthmodulated (PWM) voltage source inverters (often referred to as MV/LVlink converters) to derive the LVDC distribution voltage from aconventional MVAC distribution system. The LVDC is distributed using aring main to provide redundancy then via fault current rated DCCBs toZPSUs and other electrical loads.

Unlike in a conventional ac current distribution system, a dc currentdistribution system will not experience regular current line reversals.The DCCBs must therefore interrupt fault current by electromechanicallycausing contacts to open, thereby causing arc voltage to be generatedbetween the contacts. The arc voltage opposes a system voltage that isthe sum of the power supply voltage source that causes the fault currentto flow and the inductively generated voltage that opposes any reductionin the fault current. This allows the arc voltage to reduce the faultcurrent and eventually completely interrupt it. As the fault currentapproaches final interruption, the arc voltage will experience atransient increase that is known to stress components that are connectedto the SSDS and which generates electromagnetic interference (EMI). Thiscomponent stress is exacerbated by the summation of the DCCB transientarc voltage and the recovery of the SSDS distribution voltage thatresults from the interruption of the fault current that flows in thepower supply voltage source. It is known to apply surge arresters andsnubbers to such power distribution systems to reduce the transient arcvoltages and EMI.

It is also known to use hybrid DCCBs that use a series connectedcombination of power electronic switching devices andelectromagnetically actuated electrical contacts such that the powerelectronic switching devices rapidly switch off, a surge arrester andsnubber moderate the resultant voltage transient and the electricalcontacts are opened following the interruption of the fault current.

Linear regulator de power supply units use a technique called “foldback”to limit regulator power device dissipation during short circuit loadconditions. A foldback system typically comprises an output currentlimiting regulator whose reference is output voltage-dependent. If loadimpedance drops below a particular threshold, the initial action of thecurrent limiting regulator is to cause the output voltage to reduce,followed by a regenerative action that serves to limit the outputcurrent and voltage to suitable low levels and limit regulator powerdevice dissipation.

SUMMARY OF THE INVENTION

The present invention provides a power distribution system comprising:

-   -   a first power generation system including at least one power        source for supplying power to a first service distribution        system that includes:        -   at least one dc distribution busbar for carrying a            distribution voltage and a distribution current, and        -   at least one switchboard that includes protective switchgear            with contacts;    -   a zonal power distribution sub-system including:        -   a zonal power supply unit for supplying power to at least            one electrical load, and        -   a zonal energy store connected to the at least one            switchboard of the first service distribution system for            supplying power to the zonal power supply unit;    -   wherein the at least one power source is regulated according to        a power source foldback and stabilizing characteristic and a        power source starting characteristic, and wherein the at least        one electrical load is regulated according to a load shedding        and stabilizing characteristic;    -   wherein the contacts of the protective switchgear are made to        open only when the distribution voltage and the distribution        current have been reduced to acceptable levels by the        interaction of the power source foldback and stabilizing        characteristic with one of (a) a fault that causes an        excessively low impedance to be connected across the        distribution voltage, (b) an overriding inter-tripping command        that is automatically generated within the power distribution        system, (c) an overriding inter-tripping command that is        manually generated within the power distribution system, and (d)        an overriding inter-tripping command that is generated remotely;        and    -   wherein the contacts of the protective switchgear are made to        close only when the polarity of the voltage across the contacts        is such that any transient or inrush currents will be restricted        by one of (a) the power source foldback and stabilizing        characteristic and the power source starting sequence, and (b)        the load shedding and stabilizing characteristic.

In general terms, the power distribution system includes at least onepower source whose output current is rectified or naturally produces dccurrent. The output current is preferably limited by fast acting means(such as a power converter, for example) according to the power sourcefoldback and stabilizing characteristic that initially causes maximumprospective fault current to be much larger than for a conventionalimpedance-limited case, and that subsequently causes the output currentto be commutated in a coordinated action. The power source foldback andstabilizing characteristic also facilitates current sharing betweenparallel-connected power sources by incorporating a steady state droopcomponent. Moreover, the power source foldback and stabilizingcharacteristic also facilitates the stabilization of the distributionvoltage by incorporating an appropriate transient response that issuperimposed on the steady state droop component.

When a low impedance fault is applied to the power distribution system,the action of the at least one power source is eventually to cause thefault current to be interrupted according to a foldback method. Whilethe fault current interruption progresses, sensors associated with theprotective switchgear in the switchboard that is associated with thepath of the fault current, and an associated electronic processor,detect the fault and determine that protective switchgear must beopened. Once the fault has been interrupted, the electronic processordetermines that this is the case and instructs the protective switchgear(optionally an off-load type switchgear) to open.

A number of electrical loads may be connected to the power distributionsystem and these are all electronically regulated by fast acting means(such as a power converter, for example) according to a particular loadshedding and stabilizing characteristic that causes load current to beremoved in a manner that is coordinated with the above-mentionedcommutation of the output current of the at least one power source. Whenthe protective switchgear is opened, the effect of the removal of loadcurrent is such as to allow the output voltage of the at least one powersource to recover according to a foldback method. The recovery of thisoutput voltage initiates the re-application of the electrical loadsaccording to a load shedding method. The load shedding and stabilizationcharacteristic also causes a particular transient response of loadcurrent with respect to the supply voltage to be superimposed on thesteady state response of the electrical loads.

The above method can also be initiated by other failure modes that aredetected by other sensors and the electronic processor, or by specificelectronic processor commands, by a method of inter-tripping. Allaspects of the method are preferably programmable by suitable means. Thepower distribution system does not require serial communication betweenthe at least one power source, protective switchgear and electricalloads in order to operate because distributed intelligence and aneffective means of communication is provided by the power distributionsystem itself. All the component parts of the power distribution systemcan operate automatically and autonomously. However, if serialcommunication is provided then the power distribution system is capableof benefiting from increasing intelligence and automation. Local manualcontrols may be provided for all component parts.

Power is distributed through the power distribution system and inparticular to one or more zonal power distribution sub-systems thatincorporate zonal energy stores. These zonal energy stores areinherently capable of supporting reversible power flow. The zonal energystores may be charged from the first service distribution system inorder to provide continuous power to electrical loads connected to thezonal power supply unit despite interruptions in the distributionvoltage. However, the zonal energy stores may also supply power back tothe first power distribution system to assist in the stabilization ofthe distribution voltage.

Power converters are preferably employed to adapt the outputs of allpower sources to the appropriate dc distribution voltage and to providefault current limitation. This allows for greater design freedom and theoptimisation of power generation equipment. All electrical loads arealso preferably conditioned by power converters, which actively assistin stabilizing the distribution voltage and limit fault currents andswitching transients. The power distribution system, and in particularthe protective switchgear, need only be optimised for its continuousrunning loads since fault currents and switching transients are limitedby active means.

The power distribution system preferably has a highly redundant andreconfigurable topology to provide graceful degradation. This isparticularly important if the power distribution system is used on navalships or submarines where it must continue to provide power to criticalsystems even if component parts are damaged. The stability of the powerdistribution system means that heavy pulsed loads (such as kineticenergy (KE) projectile and unmanned aerial vehicle (UAV) launchers, forexample) can be supplied. The proportion of power drawn by anyelectrical load may be continuously adjusted to optimise efficiency andfacilitate “bump-free” transitions between single island and multipleisland configurations. The distribution voltage may be lower than inconventional power distribution systems, with respect to total installedpower generation capacity, thereby reducing insulation requirements andmaximizing power density.

The operation of the power distribution system is essentially automaticbut may incorporate manual reversionary modes. All critical componentparts are preferably intelligent and autonomous. The intelligence may besummarized as follows.

When a power source (such as a generator, for example) is started, itsassociated power converter regulates the output voltage and ramps it tojust below a desired output voltage. The associated protectiveswitchgear detects this state of readiness and closes. The power sourceis safe against backfeed, senses that it is on-line and transits to itsspecified output characteristic.

When a serious overload occurs, the fault current is limited by thepower source foldback and stabilizing characteristic. The protectiveswitchgear rapidly locates and classifies the fault by reference to itssensors. If the fault persists, the power source foldback andstabilizing characteristic causes the output voltage to reduce. Allelectrical loads shed or revert to zonal energy stores. The protectiveswitchgear senses that it is safe to open the affected output. When thefault is removed, all other electrical loads being shed or havingreverted to zonal energy stores, the output voltage recovers accordingto the power source foldback and stabilizing characteristic and the loadshedding and stabilizing characteristic is removed.

The power distribution system may further comprise a second powergeneration system including at least one power source for supplyingpower to a second service distribution system. The second servicedistribution system preferably includes at least one dc distributionbusbar for carrying a distribution voltage and a distribution current,and at least one switchboard that includes protective switchgear withcontacts. The zonal energy store of the zonal power distribution systemcan be connected to the at least one switchboard of the second servicedistribution system. In this way, the zonal energy store can be suppliedfrom the first service distribution system and/or the second servicedistribution system.

In the case where the power distribution system forms part of a marinepower and propulsion system then it may further include a firstpropulsion drive system, a second propulsion drive system, a firstpropulsion power generation system including at least one power sourceand a second propulsion power generation system including at least onepower source. Each of the first and second propulsion drive systems mayinclude a propeller that is driven by a propulsion motor whose powerflow is regulated by a power converter. Each of the first and secondpropulsion power generation systems may include a prime mover (such as aturbine, for example) that drives a generator to supply power to a powerconverter.

The first propulsion drive system preferably has three power supplyinputs, each input being selectable (by means of a system of manuallyconnected links or other suitable means of isolation, for example). Thefirst power supply input may be connected to the first propulsion powergeneration system, the second power supply input may be connected to thesecond propulsion power generation system, and the third power supplyinput may be connected to the at least one switchboard of the firstservice distribution generation system. The first propulsion drivesystem may therefore be supplied with power by the first propulsionpower generation system through the first power supply input and/or bythe second propulsion power generation system through the second powersupply input. Power can also be supplied to the first propulsion drivesystem from the first service distribution system through the thirdpower supply input. If the first propulsion drive system is operated ina regenerative mode then it can also be used to supply power to thefirst service distribution system.

The second propulsion drive system preferably has three power supplyinputs, each being selectable (by means of a system of manuallyconnected links or other suitable means of isolation, for example). Thefirst power supply input may be connected to the first propulsion powergeneration system, the second power supply input may be connected to thesecond propulsion power generation system, and the third power supplyinput may be connected to the at least one switchboard of the secondservice power distribution system. The second propulsion drive systemmay therefore be supplied with power by the first propulsion powergeneration system through the first power supply input and/or by thesecond propulsion power generation system through the second powersupply input. Power can also be supplied to the second propulsion drivesystem from the second service distribution system through the thirdpower supply input. If the second propulsion drive system is operated ina regenerative mode then it can also be used to supply power to thesecond service distribution system.

If the switchboards of the first and second service distributionssystems are interconnected or cross linked then further redundancybetween the two sides of the power distribution system can be provided.

The first propulsion power generation system preferably has first andsecond power supply outputs, each being selectable (by means of a systemof manually connected links or other suitable means of isolation, forexample). The first power supply output may be connected to the firstpower supply input of the first propulsion drive system and the secondpower supply output may be connected to the first power supply input ofthe second propulsion drive system. The first propulsion powergeneration system may therefore supply power to the first propulsiondrive system through the first power supply output and/or the secondpropulsion drive system through the second power supply output. If thefirst propulsion power generation system has a third power supply outputthat is selectable and is connected to the at least one switchboard ofthe first service distribution system then power can also be supplied tothe first service distribution system.

The second propulsion power generation system preferably has first andsecond power supply outputs, each being selectable (by means of a systemof manually connected links or other suitable means of isolation, forexample). The first power supply output may be connected to the secondpower supply input of the first propulsion drive system and the secondpower supply output may be connected to the second power supply input ofthe second propulsion drive system. The second propulsion powergeneration system may therefore supply power to the first propulsiondrive system through the first power supply output and/or the secondpropulsion drive system through the second power supply output. If thesecond propulsion power generation system has a third power supplyoutput that is selectable and is connected to the at least oneswitchboard of the second service distribution system then power canalso be supplied to the second service distribution system.

The power distribution system may be configured such that power can besupplied to the first service distribution system through the at leastone switchboard by one or more of the following: the first powergeneration system, the zonal energy store of the zonal powerdistribution sub-system, a propulsion drive system operating in aregenerative mode, a propulsion power generation system, and a remotepower supply system such as a shore-based power supply, for example.This provides a considerable degree of redundancy.

The at least one power source of the first power generation system ispreferably one or more of the following: a diesel generator, a gasturbine generator, a steam turbine generator, a combined cycle gas andsteam turbine generator, a closed cycle (non-air breathing) dieselgenerator, a battery, a fuel cell, a flow cell, a flywheel generator, asuper-capacitor (i.e. a capacitor with extremely high capacity andcapacitive energy density), and a superconducting magnetic energy store.This should not be considered an exhaustive list and it will be readilyunderstood that other power sources can be used.

The at least one power source of the first power generation system ispreferably connected to the at least one switchboard of the firstservice distribution system by a power converter.

The zonal energy store of the zonal power distribution sub-system isalso preferably connected to the at least one switchboard of the firstservice distribution system by a power converter. In both cases, thepower converter is preferably a pulse width modulated dc/dc converter.

The dc/dc converter is preferably polarized as a step-up chopper whenpower flows from the first service distribution system into the zonalenergy store of the zonal power distribution sub-system, and the dc/dcconverter is polarized as a step-down chopper when power flows from thezonal energy store of the zonal power distribution sub-system into thefirst service distribution system.

The output voltage and output current of the at least one power sourceof the first power generation system is preferably regulated such thatthe current flow is uni-directional. Further regulation provides that asteady state output voltage can be the sum of an off load bus voltagesetpoint and a steady state droop component that is proportional to loadcurrent such that the steady state output voltage is in accordance witha steady state load line. Transient load current variations about asteady state loading point preferably cause the output voltage to followa transient load line whose gradient is less than the gradient of thesteady state load line. Steady state current is preferably limited to aparticular level. If load current transiently exceeds the steady statecurrent limit and approaches, but does not exceed, a particulartransient current limit level, then the output voltage will preferablytransiently reduce with respect to the steady state load line and willrecover to the steady state load line when the steady state currentreduces below the steady state current limit. If load currentcontinuously exceeds the steady state current limit, or exceeds theparticular transient current limit level, then foldback is preferablyapplied such that the output voltage and the output current reduce tozero according to a regenerative process, and output voltage and outputcurrent remain at zero until load impedance has increased beyond aparticular level. If load impedance increases beyond the particularlevel then load voltage initially partially recovers and then ispreferably ramped up to a desired operating point.

The load voltage can be ramped up to the desired operating pointaccording to a time-variable ramp rate that is specified to minimizeresultant voltage transients within the power distribution system.

The distribution voltage is preferably stabilized by a transient loadline function of the power source foldback and stabilizingcharacteristic and by a limitation of rate of change of load currentfunction of the load shedding and stabilizing characteristic.

The first power generation system may include a plurality ofparallel-connected power sources for supplying power to a first servicedistribution system. In this case, the steady state current sharing ofthe plurality of power sources can be coordinated by a steady statedroop function of the power source foldback and stabilizingcharacteristic of each power source. The transient current sharing ofthe plurality of power sources can be coordinated by a transient loadline function of the power source foldback and stabilizingcharacteristic of each power source.

The at least one switchgear may include distribution busbars, incomingbusbars, outgoing busbars. At least one electromechanically actuatedoff-load double pole switch is preferable connected to the distributionbusbars. The at least one switchgear is preferable controlled by anelectronic control system that includes an electronic processor, currentsensor on all distribution busbars, incoming busbars and outgoingbusbars, voltage sensors of all distribution busbars, incoming busbarsand outgoing busbars, inter-tripping inputs, inter-tripping outputs anddrivers for switch actuators. The electronic control system may alsoinclude a local operator interface and a remote control interface.

The present invention further provides a method of controlling a powerdistribution system comprising:

-   -   a first power generation system including at least one power        source for supplying power to a first service distribution        system that includes:        -   at least one dc distribution busbar for carrying a            distribution voltage and a distribution current, and        -   at least one switchboard that includes protective switchgear            with contacts;    -   a zonal power distribution sub-system including:        -   a zonal power supply unit for supplying power to at least            one electrical load, and        -   a zonal energy store connected to the at least one            switchboard of the first service distribution system for            supplying power to the zonal power supply unit;    -   the method comprising the steps of:    -   regulating the at least one power source according to a power        source foldback and stabilizing characteristic and a power        source starting characteristic; and    -   regulating the at least one electrical load according to a load        shedding and stabilizing characteristic;    -   wherein the contacts of the protective switchgear are made to        open only when the distribution voltage and the distribution        current have been reduced to acceptable levels by the        interaction of the power source foldback and stabilizing        characteristic with one of (a) a fault that causes an        excessively low impedance to be connected across the        distribution voltage, (b) an overriding inter-tripping command        that is automatically generated within the power distribution        system, (c) an overriding inter-tripping command that is        manually generated within the power distribution system, and (d)        an overriding inter-tripping command that is generated remotely;        and    -   wherein the contacts of the protective switchgear are made to        close only when the polarity of the voltage across the contacts        is such that any transient or inrush currents will be restricted        by one of (a) the power source foldback and stabilizing        characteristic and the power source starting sequence, and (b)        the load shedding and stabilizing characteristic.

The distribution voltage is preferably stabilized by a transient loadline function of the power source foldback and stabilizingcharacteristic and by a limitation of rate of change of load currentfunction of the load shedding and stabilizing characteristic.

The power distribution system preferably has a power source startingsequence where:

-   -   the off load bus voltage setpoint of the power source foldback        and stabilizing characteristic is initially set to zero;    -   the at least one power source of the first power generation        system detects a need to start supply power by sensing one        of (a) the presence of distribution voltage resulting from the        closure of the at least one switchboard of the first service        distribution system, (b) an overriding start command that is        automatically generated within the power distribution        system, (c) an overriding start command that is manually        generated within the power distribution system, and (d) an        overriding start command that is generated remotely;    -   upon detecting a need to start supplying power, the at least one        power source is started and the off load bus voltage setpoint of        the power source foldback and stabilizing characteristic is        ramped up to a desired operating point.

The load voltage may be ramped up to the desired operating pointaccording to a time-variable ramp rate that is specified according tothe dynamic capability of the at least one power source and the need toallow the at least one power source to progressively supply anincreasing proportion of the total load current in the powerdistribution system to minimize resultant voltage transients within thepower distribution system.

The load shedding and stabilizing characteristic preferably includes thestep of regulating the load current according to a current limit suchthat load current is permitted to attain any desired value but is alwayssubject to overriding regulator functions that:

-   -   limit the rate of change of load current resulting from        distribution voltage transients; and    -   oppose any action that would otherwise cause load current to        exceed the current limit;    -   and where the current limit:    -   is adjustable up to and not exceeding a particular value of        current limit;    -   is held constant when supply voltage exceeds a load shed        threshold;    -   is progressively reduced as the supply voltage is reduced below        the load shed threshold and at all levels of supply voltage        above an absolute minimum loaded voltage;    -   is set to zero when the supply voltage is less than the absolute        minimum loaded voltage;    -   is set to zero when the supply voltage increases from a value        less than the absolute minimum loaded voltage and up to a        particular value; and    -   is progressively increased as the supply voltage is increased.

All the parameters of the power source foldback and stabilizingcharacteristic, the load shedding and stabilizing characteristic and thepower source starting sequence are preferably programmable by anyconvenient means.

The power distribution system preferably has an over current protectionsequence in a situation where a low impedance fault occurs in the powerdistribution system, the over current protection sequence including thesteps of:

-   -   locating the low impedance fault within the power distribution        system;    -   limiting the fault current and distribution voltage by applying        the power source foldback and stabilizing characteristic;    -   limiting the load current by applying the load shedding and        stabilizing characteristic;    -   detecting fault interruption;    -   opening the contacts of the protective switchgear;    -   waiting for the partial recovery of the distribution voltage        caused by the opening of the contacts of the protective        switchgear;    -   waiting for the full recovery of the distribution voltage caused        by the application of the power source foldback and stabilizing        characteristic; and    -   waiting for the re-application of the load current caused by the        application of the load shedding and stabilizing characteristic.

The power distribution system preferably has a general purposeprotective or power management sequence including the steps of:

-   -   detecting a fault condition or the establishment of any power        management condition that requires the contacts of the        protective switchgear to be opened;    -   generating an overriding inter-tripping command;    -   limiting the distribution voltage by applying the power source        foldback and stabilizing characteristic;    -   limiting the load current by applying the load shedding and        stabilizing characteristic;    -   detecting a load current interruption;    -   opening the contacts of the protective switchgear;    -   waiting for the partial recovery of the distribution voltage        caused by the opening of the contacts of the protective        switchgear;    -   waiting for the full recovery of the distribution voltage caused        by the application of the power source foldback and stabilizing        characteristic; and    -   waiting for the re-application of the load current caused by the        application of the load shedding and stabilizing characteristic.

The zonal energy store of the zonal power distribution sub-system mayreceive power from, or supply power to, the first service distributionsystem. The power is preferably regulated for the purpose of re-chargingthe zonal energy store, supplying power to the zonal power supply of thezonal power distribution sub-system, supplying power to the firstservice distribution system, providing a bulk energy store, supplyingpower continuously for any purpose, supplying power transiently toassist in stabilizing the distribution voltage, supplying powertransiently to support other power sources that have a poor transientresponse, providing isolation between the zonal energy store and thefirst service distribution system to allow the zonal power supply tooperate independently when the first service distribution system issubject to a failure, or allowing the first service distribution systemto operate independently of the zonal energy store when the zonal energystore or the zonal power supply is subject to a failure.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will now be described, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a conventional marine power andpropulsion system that employs full electric propulsion (FEP);

FIG. 2 is a schematic diagram of a power distribution system accordingto the present invention;

FIG. 3 is a diagram showing the output voltage versus output currentcharacteristic of a power source forming part of the power distributionsystem of FIG. 2;

FIG. 4 is a diagram showing the load current versus supply voltagecharacteristic of an electrical load forming part of the powerdistribution system of FIG. 2;

FIG. 5 is a schematic diagram of protective switchgear forming part ofthe power distribution system of FIG. 2; and

FIG. 6 is a diagram showing the output voltage versus output currentcharacteristic of a power source forming part of the power distributionsystem of FIG. 2, where the effect of incomplete load shedding is shown.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although the following description is directed to a power distributionsystem for marine applications, and in particular to a powerdistribution system that is highly suitable for naval ships andsubmarines, it will be readily understood that a similar topology andcontrol method can be used in other sorts of power distribution systemssuch as land-based or aircraft-based systems, for example.

The basic topology of a power distribution system according to thepresent invention will now be described with reference to FIG. 2. Itshould be appreciated that all of the inputs, outputs, terminals andinterconnections of FIG. 2 are of double pole direct current type.

A first Propulsion Power Generation System (PPGS) includes a turbine 1that drives a generator 2 to supply power to an output converter 3.Similarly, a second PPGS includes a turbine 10 that drives a generator11 to supply power to an output converter 12. A first Propulsion DriveSystem (PDS) includes a propeller 6 that is driven by a propulsion motor5 whose power flow is regulated by a propulsion converter 4. Similarly,a second PDS comprises a propeller 9 that is driven by a propulsionmotor 8 whose power flow is regulated by a propulsion converter 7. Theoutput converters 3 and 12 each have three sets of output terminals andthe propulsion converters 4 and 7 each have three sets of inputterminals, all such terminals have linking arrangements that will bedescribed in more detail below.

The generators 2 and 11 may be implemented using any number of differentcomponents and the choice will normally be influenced by the type ofprime mover. For example, for conventional gas turbines the generatorsmay be implemented using the Two Pole Turbo range of solid polesynchronous alternators supplied by Converteam Ltd of Boughton Road,Rugby, Warwickshire, CV21 1BU, United Kingdom (hereafter referred to as“Converteam”). For higher speed turbines the generators may beimplemented using Motor Grand Vitesse (induction generator variant)supplied by Converteam. Finally, for lower speed turbines and dieselengines the generators may be implemented using the ALPHA, BETA andGAMMA ranges of salient pole synchronous machines supplied byConverteam.

The propulsion converters 4 and 7 may be implemented using VDM2500,MV7000 and MV3000 ranges of pulse width modulated (PWM) voltage sourceinverters supplied by Converteam.

The propulsion motors 5 and 8 may be implemented using AdvancedInduction Motors supplied by Converteam.

A first Ship Service Power Generation System (SSPGS) comprises a dieselengine 13 that drives a generator 14 to supply power to an outputconverter 15. Similarly, a second SSPGS comprises a diesel engine 17that drives a generator 18 to supply power to an output converter 19. Aplurality of zonal power distribution sub-systems each comprise a ZonalPower Supply Unit (ZPSU) 23 that feeds power into a conventional lowvoltage (LV) distribution system and draws power from a Zonal EnergyStore (ZES) 22 that is connected to a first dc/dc converter 21 and asecond dc/dc converter 24. In FIG. 2, three zonal power distributionsub-systems are shown, but more or fewer may be used in practice.

The generators 14 and 18 may be implemented using the ALPHA, BETA andGAMMA ranges of salient pole synchronous machines supplied byConverteam.

The output converters 15 and 19 may be implemented using VDM2500, MV7000and MV3000 ranges of pulse width modulated (PWM) voltage sourceinverters supplied by Converteam.

The dc/dc converters 21 and 24 and the ZPSU 23 may be implemented usingDELTA modules supplied by Converteam Ltd.

A first Ship Service Distribution System (SSDS) includes dc distributionbusbars 25, 27 and 29, interposed by switchboards 16, 26 and 28 that aredescribed in more details below. Similarly, a second SSDS includes dcdistribution busbars 30, 32 and 34, interposed by switchboards 20, 31and 33. In FIG. 2, the first and second SSDS each comprise threeswitchboards, this quantity being associated with the quantity of zonalpower distribution sub-systems shown, but more or fewer may be used inpractice. A shore supply 39 is connected to a shore supply outputconverter 38.

The various component parts of the power distribution system areinterconnected as follows:

The first input of propulsion converter 4 is connected to the firstoutput of output converter 3 by interconnection 41.

The second input of propulsion converter 4 is connected to the firstoutput of output converter 12 by interconnection 43.

The third input of propulsion converter 4 is connected to the firstoutput of switchboard 16 by interconnection 42.

The first input of propulsion converter 7 is connected to the secondoutput of output converter 3 by interconnection 46.

The second input of propulsion converter 7 is connected to the secondoutput of output converter 12 by interconnection 44.

The third input of propulsion converter 7 is connected to the firstoutput of switchboard 20 by interconnection 45.

The output of output converter 15 is connected to the first input ofswitchboard 16.

The output of output converter 19 is connected to the first input ofswitchboard 20.

The third output of the output converter 3 is connected to the secondinput of switchboard 16 by interconnection 47.

The third output of the output converter 12 is connected to the secondinput of switchboard 20 by interconnection 48.

The supply terminals of dc/dc converter 21 are connected to the secondoutput of switchboard 16.

The supply terminals of dc/dc converter 24 are connected to the secondoutput of switchboard 20.

The cross linking terminals of switchboard 16 are linked to the crosslinking terminals of switchboard 20 by cross link 35.

The cross linking terminals of switchboard 26 are linked to the crosslinking terminals of switchboard 31 by cross link 36, and to the outputterminals of output converter 38 by shore supply link 37.

Similarly, switchboards 26, 28, 31 and 33 are connected to additionalzonal power distribution sub-systems.

Similarly, switchboards 28 and 33 are connected by cross link 40.

It will be readily appreciated that the terms “input” and “output” referto the normal direction of power flow but it may be the case that powerwill flow out of an input and into an output in certain circumstances.For example, power will normally flow through interconnection 42 fromthe first output of the switchboard 16 to the third input of thepropulsion converter 4. However, if the propulsion motor 5 were to beoperated in a regenerative mode than power could flow throughinterconnection 42 from the third input of the propulsion converter 4 tothe first output of the switchboard 16.

The process by which current that flows in a short circuit or lowresistance fault is interrupted will now be described with reference toFIG. 3. For the purpose of this description, a fault is considered tooccur within dc/dc converter 21. A variable load current flows prior tothe fault and the protective action of a power source comprising dieselengine 13, generator 14 and output converter 15 is considered tointerrupt the fault. However, it should be noted that the followingdescription is equally applicable to faults that may occur anywherewithin the power distribution system and to any power source that feedsthe power distribution system.

FIG. 3 shows the characteristic of output voltage against variableoutput current for a power source comprising the diesel engine 13, thegenerator 14 and the output converter 15. The diesel engine 13 isgoverned by a conventional governor and is set to run at any convenientrotational speed. The generator 14 is regulated by a conventionalAutomatic Voltage Regulator (AVR) and generator output voltage is set toany convenient level. The output converter 15 is regulated by aregulator (not shown) according to a foldback and stabilizingcharacteristic shown in FIG. 3; this characteristic being substantiallyindependent of the actions of the conventional governor and AVR. Anyconvenient type of regulator may be used for the purpose of regulatingthe output converter 15, but a programmable digital regulator would bethe preferred type.

An off load bus voltage setpoint (the “setpoint”) is provided to theregulator of the output converter 15 and all other regions of thefoldback and stabilizing characteristic are derived from this. In normaloperation as the load current in the dc/dc converter 21 is progressivelyincreased, the output current in the output converter 15 alsoprogressively increases and the output voltage of the output converter15 reduces according to a steady state load line which droops withrespect to the setpoint according to the steady state droop. The steadystate droop may be proportional to output current or it may conform toany other suitable characteristic. By fast acting regulator action,transient changes in the load current and the equivalent output currentwill cause the output voltage to be displaced from the steady state loadline. If the steady state average values of output current and voltageare at the steady state loading point shown in FIG. 3, and transientperturbations of output current occur about the steady state loadingpoint, the output voltage will conform to the transient load line aboutthe example steady state loading point. The transient load line isrepresented in FIG. 3 by the dashed arrows extending on both sides ofthe steady state loading point and it may be displaced from the steadystate load line by a degree that may be proportional to output currenttransient perturbation, or it may conform to any other suitablecharacteristic.

The output current is subject to a fast acting overriding transientcurrent limit such that the output voltage is reduced in order toprevent the instantaneous level of output current from exceeding thetransient current limit. Load current is also subject to a steady statecurrent limit such that the output voltage is reduced in order toprevent the steady state average level of output current from exceedingthe steady state current limit. If the load resistance falls below thatof the gradient of the steady state foldback characteristic thenfoldback is applied. This means that the transient output current limitis reduced to a level that is output voltage dependent, this dependencybeing such as to cause the regenerative reduction of output current andoutput voltage. The regenerative action converges upon a point where theoutput current and the output voltage have been reduced to approximatelyzero. In practice, when a fault is applied at a time when the steadystate output current is approaching the steady state current limit, theoutput current will rapidly increase until the transient current limitaction causes the output voltage to collapse. Foldback is then appliedand the fault is said to be interrupted when the output current and theoutput voltage approach zero. The practical foldback characteristic issuch that the minimum level of transient current limit is a small,non-zero, level for reasons that will now be explained.

At the point when the fault current has been practically interrupted, asmall, non-zero, level of output current will flow into the loadrepresented by dc/dc converter 21 through protective switchgear inswitchboard 16. If this switchgear is opened and is able to interruptthe small, non-zero, current level then the distribution voltage willincrease, providing no other load is present.

In the case where no other load is present, the fault is known to becleared when the distribution voltage increases. Since the loadresistance has increased beyond that of the gradient of the steady statefoldback characteristic when distribution voltage increases, theregenerative action of the foldback is released and the distributionvoltage returns to the level set by the setpoint.

In the case where an additional load is connected in parallel with thedc/dc converter 21, the foldback will not be released if the resistancepresented by this load is less than that of the gradient of the steadystate foldback characteristic. When multiple loads are connected inparallel, and they must be returned to operation after faultinterruption and clearance, it is necessary for them to shed load whenthe distribution voltage is reduced below normal working levels in orderto permit the foldback to be released.

A benefit of the foldback and stabilizing characteristic of the presentinvention is that protective switchgear is called upon to operate atnear zero current—zero voltage conditions, thus avoiding the need toemploy conventional switchgear in which complex arc control apparatus isused to oppose direct current. The foldback and stabilizingcharacteristic also facilitates the parallel connection of multiple anddisparate types of power sources since the respective power sourceoutput characteristics may be controlled by fast acting regulatorfunctions and power electronics. In this way the natural impedance andresponse characteristics of disparate generator types can be decoupled.

The associated load shedding and stabilizing characteristic will now bedescribed in more detail with reference to FIG. 4. For the purpose ofthis description, the same fault as was previously described withreference to FIG. 3 is considered to occur within dc/dc converter 21,while an additional load is presented by a propulsion drive comprisingthe propeller 6, the propulsion motor 5 and the propulsion converter 4.This means that the dc/dc converter 21 and the additional load areconnected in parallel. A variable load current flows prior to the faultand the protective action of a power source comprising the diesel engine13, the generator 14 and the output converter 15 is considered tointerrupt the fault.

Prior to the occurrence of the fault in the dc/dc converter 21, theoutput current of the output converter 15 is the sum of the loadcurrents drawn by the dc/dc converter 21 and the propulsion drive. Thepropulsion drive load is regulated by the regulator that controls thepropulsion converter 4. The propulsion drive load may be regulated toachieve constant propulsive power or to satisfy any other operationalrequirement, but the constant power case serves the purpose ofexplanation of the load shedding and stabilizing characteristic well. Ifconstant propulsive power is drawn by the propulsion converter 4 thenits load current will be approximately inversely proportional to itssupply voltage. (A reduction in supply voltage is associated with anincrease in load current in order to maintain constant power.) It isknown for propulsion converters to have a supply current limit functionthat prevents the rated current limit from being exceeded when thepropulsion converter load is at rated power and supply voltage isreduced below the rated minimum. FIG. 4 shows three curves of supplyvoltage and load current when at constant power. The uppermost constantpower curve is specific to “constant rated power” and this intersectswith the lines of rated current limit and minimum voltage for ratedload. If the supply voltage is reduced below this intersect then loadcurrent is initially held constant at the rated current limit level andpropulsion load power is reduced pro rata with the supply voltage. Ifthe supply voltage is reduced below a load shed threshold then loadshedding is said to be applied and the load current deviates from theknown rated current limit and is regulated according to a first supplyvoltage dependent current limit. This means that the propulsion loadpower is reduced according to a higher order law than proportionalitywith supply voltage. Load shedding is applied until the supply voltageis reduced to a particular absolute minimum loaded voltage, below whichit is considered that the distribution voltage has collapsed as a resultof a fault and under voltage tripping occurs.

When the under voltage trip occurs, the propulsion drive assumes acontrolled state where its supply current is switched off by thepropulsion converter 4 and the propulsion drive remains under control toenable a rapid re-start once fault conditions are cleared. Thepropulsion drive remains in this controlled state until the supplyvoltage has increased to a particular level where load shedding isreleased and the load current is permitted to increase according to asecond supply voltage dependent current limit until normal (non currentlimited) working is resumed.

The load shedding characteristic described above is beneficial inallowing a propulsion drive, or any other load, to draw power from asupply, usefully, providing supply voltage is within prescribed limits.The “constant reduced power curve” shows how the supply current would befree from limitation until the supply voltage was reduced to the pointwhere the curve of load current intersects the lines of rated currentlimit and load shed threshold. The “constant reduced power at permittedlow supply voltage” curve shows how supply current would be free fromlimitation when a propulsion drive was operated at relatively low outputpower with a supply voltage significantly lower than the range of“voltages for rated load”. The ability of a medium voltage drive with,for example, 5 kV nominal supply rating to operate from a SSDS powersupply with a nominal supply voltage rating of, for example, 750V ispermitted by the present invention and provides a distinct technicaladvantage over conventional power distribution systems. Furthermore, theload shedding characteristic below “absolute minimum loaded voltage” inthe present invention is beneficial in allowing the power sourcefoldback to be released, providing the protective switchgear isinstructed to open in order to clear the fault.

Another aspect of the load shedding and stabilizing characteristic isthe regulation of a load to minimize the rate of change of load currentthat may arise in response to a transient variation in supply voltage orother perturbation within the drive system. In this case, if a load isoperating at the “example steady state loading point” (at a supplyvoltage between minimum and nominal levels) and supply voltagefluctuations occur, the load current will deviate from the constantpower curve and will adopt the “transient load line about example steadystate loading point”. The skilled reader will be aware that the minimumdynamic load resistance that can be applied to a power source, having aparticular dynamic source resistance, without incurring instability, isa function of the dynamic source resistance, (i.e., if dynamic loadresistance is too low, distribution voltage cannot be stabilized). Theload shedding and stabilizing characteristic of the present inventiontherefore provides the necessary means of ensuring that dynamic loadresistance is sufficiently large, with respect to dynamic sourceresistance, to ensure that distribution voltage stability is achieved.It should be noted that the terms “dynamic source resistance” and“dynamic load resistance” do not imply that actual physical resistanceand consequential power dissipation must be employed in order tostabilize the distribution voltage. To the contrary, the terms refer toclassical control functions that mimic the effects and transferfunctions of equivalent passive components. The load shedding andstabilizing characteristic also facilitates the parallel connection ofmore than one load to a point of common coupling and load sharing iseffective over a wide range of supply voltages. When groups of loads andpower sources are connected in parallel, the total dynamic loadresistance that is experienced by the group of power sources is theparallel combination of the load characteristics and these may beprogrammed to achieve stable operation with the maximum designed dynamicsource resistance. As parallel connected power sources are added to thedistribution network, dynamic source resistance is reduced and stabilitymargins will increase.

In the power distribution topology of FIG. 2, the dc/dc converters 21and 24 may be routinely called upon to operate as loads for part of thetime, and as power sources for the remainder of the time. When charginga ZES 22 and/or feeding power to a ZPSU 23, the regulators of the dc/dcconverters 21 and 24 must comply with the above-mentioned load sheddingand stabilizing characteristic. When a ZES 22 is feeding power into aSSDS via dc/dc converters 21 and 24, the regulators of the dc/dcconverters 21 and 24 must comply with the above-mentioned foldback andstabilizing characteristic. Stepless bidirectional transfer is requiredbetween these characteristics. Propulsion drives may also be given thecapability for bidirectional power flow.

When a power source is not required or permitted to receive power fromanother power source, the anti-backfeed region of the foldback andstabilizing characteristic shown in FIG. 3 is employed. By this means, apre-existing supply voltage may be connected to the output of a powersource and its output voltage may be ramped up until the anti-backfeedregion of the characteristic is cleared and the power source outputspower. A benefit of the anti-backfeed function is that switchgear forthe SSDS may be closed onto a power source output without sufferinginrush current or requiring the power source to have the complexsynchronizing apparatus normally associated with alternating currentdistribution systems.

The operation of the protective switchgear within switchboards 16, 26,28, 20, 31 and 33 will now be described with reference to FIG. 5. Itshould be noted that FIG. 5 is shown in full double pole format ratherthan single line format used in FIG. 1 for reasons that will bedescribed in more detail below. The functionality of the switchboards16, 26, 28, 30, 31 and 33 is in accordance with a generic process andthis generic functionality will be described without detailed referenceto the exact circuit within any particular switchboard. The detailedcircuitry of particular switchboards differs from that of the genericswitchboard shown in FIG. 5 only with respect to the number of switchedinputs and outputs. It will be appreciated that a switchboard could beproduced with any convenient number of inputs and outputs.

The generic switchboard of FIG. 5 includes a plurality of powerterminals. Half of the power terminals (namely those labelled 101, 105,108, 110 and 112) are associated with the positive (+) pole of thedirect current system. The other half of the power terminals (namelythose labelled 102, 106, 107, 109 and 111) are associated with thenegative (−) pole of the direct current system.

Two distribution busbars are also provided. The first busbar 103 isassociated with the positive pole of the direct current system and thesecond busbar 104 is associated with the negative pole of the directcurrent system. A number of individual links connect the power terminalsto the first and second busbars 103 and 104. Half of the links (namelythose labelled 113, 117 and 116) are associated with the positive poleof the direct current system. The other half of the links (namely thoselabelled 114, 118 and 115) are associated with the negative pole of thedirect current system.

The switchboard includes two double pole motor driven switches 119 and120 and a control system. The control system includes an electronicprocessor 139, a first series of current sensors 129, 137, 132, 134 and136 associated with the positive pole of the direct current system, asecond series of current sensors 130, 138, 131, 133 and 135 associatedwith the negative pole of the direct current system, a first series ofvoltage sensors 128, 121, 123 and 125 associated with the positive poleof the direct current system, a second series of voltage sensors 127,122, 124 and 126 associated with the negative pole of the direct currentsystem, a local operator interface 140, a remote control interface 143,and two inter-tripping interfaces 141 and 142 associated with switches119 and 120, respectively.

The electronic processor 139 may be implemented using a PECemicrocontroller supplied by Converteam. The switches 119 and 120 may beimplemented using proprietary motor driven molded case and chassis typeHigh Speed Direct Current Circuit Breakers, suitably interfaced to thePECe microcontroller.

The links 113, 117, 116, 114, 118 and 115 are manually bolted links thatenable the user to isolate sections of the power distribution system,but it will be appreciated that these links could be replaced byadditional double pole motor driven switches if desired.

For the purpose of this description the power terminals 111 and 112 areconsidered to be connected to a power source and power terminals 109 and110 are considered to be connected to a load.

The switchgear operating process will now be described without detailedreference to the control system. This will be described in more detailbelow.

When a low resistance fault occurs in the load, fault current flows intothe power terminal 112, through the distribution busbar 103, out of thepower terminal 110, back into the power terminal 109, through thedistribution busbar 104 and out of the power terminal 111. The circuitis completed by the double pole switches 119 and 120. The control systemis able to determine that the fault has occurred in the load andfacilitates a protective sequence by opening the double pole switch 119only when zero current flows in this switch. It will be recognized thatthe double pole switch 120 could also be opened when zero current flowsthrough this switch, but this is not preferred when other loads areconnected to the power source by the distribution busbars 103 and 104because these loads may be reliant upon the resumption of power flowfrom the power source following the interruption and clearance of thefault described above.

Some of the benefits provided by the control system will now bedescribed.

The electronic processor 139 repetitively samples the signals generatedby current sensors 129 to 138 and voltage sensors 121 to 128 with asufficiently fast response to enable the nature of a fault to bedetermined. A low resistance fault has been described and the presenceof this fault, and its exact nature, would be identified by currentsensors 136, 134, 133 and 135 in conjunction with voltage sensors 123and 124. As long as the sensing and detection of fault current isperformed before the foldback process becomes regenerative, and faultcurrent is interrupted, the current sensing is sufficient to identifythe inception and location of the fault. Once foldback has becomeregenerative, fault current has been interrupted and the distributionvoltage has collapsed to approximately zero, the electronic processor139 determines that it is safe to open a switch and switch 119 isopened. As mentioned briefly above, switch 119 would normally be openedin preference to switch 120 if the electronic processor 139 is awarethat other loads were being supplied with power prior to the inceptionof the fault. The presence of other loads would be detected by sensingload current using sensors 129, 132, 137, 130, 131 and 138.

It will be evident that such an electronic processor 139 andcomprehensive array of sensors would be able to detect a wide range ofother types of fault and that these faults may cause asymmetry ofcurrent flow in the positive and negative poles of the direct currentsystem. For example, it is known that a ground fault would cause currentto flow in only one pole. Similarly, asymmetry in voltages would occurduring a ground fault. It is therefore necessary for the control systemto be able to cause the foldback characteristic to be exercised at timeswhen power source output currents are not excessive and this isperformed by a process of inter-tripping. If the electronic processor139 determines that it is necessary to open any particular switch whenit is carrying current and distribution voltage is present, it mustfirst cause inter-tripping. In FIG. 5, inter-tripping signals 141 and142 are dedicated to such communication with the power source connectedto terminals 112 and 111, and the load connected to terminals 110 and109, respectively. If an inter-tripping signal is output to interface142, the power source that is connected to terminals 112 and 111 musthave its foldback characteristic affected, and foldback must beinitiated by the receipt of the overriding inter-tripping signal. Inanother case, a serious fault condition in a load may be such as towarrant inter-tripping. In this case, the receipt of an inter-trippingsignal 141 would be interpreted by the electronic processor 139 as beinga need to inter-trip the power source using the inter-tripping output142.

In other cases, the electronic processor 139 may generate inter-trippingsequences in response to other commands including inter alia localoperator commands generated by the local operator interface 140 andexternally generated commands communicated by the remote controlinterface 143. It will be appreciated that such an electronic processor139 could also be equipped with a global inter-tripping interface thatwould cause all power sources in the power distribution system to beinter-tripped. It will also be appreciated that the switches 119 and 120must not open spuriously and that they also must be inter-tripped andinterlocked via electronic processor 139. Such switches may have amanual reversionary operating mode and inter-tripping may be initiatedby means of an early break contact, a mechanical interlock and suitableinterfacing with the electronic processor.

It will be appreciated that a large power distribution system accordingto the present invention may incorporate many switchboards of this type.The power distribution system may be physically extensive and withsignificant capacitance between positive and negative poles. Practicalloads may also have capacitance between positive and negative poles andtheir load shedding may be imperfect. It will be appreciated that thefoldback and stabilizing characteristic described with reference to FIG.3 does not take these issues into account and the practicalcharacteristic is shown in FIG. 6. In this practical characteristic, thelocus of voltage and current when foldback is released indicates thepresence of the current that results from the recovery of thedistribution voltage after the fault has been cleared.

1. A power distribution system comprising: a first power generationsystem including at least one power source for supplying power to afirst service distribution system that includes: at least one dcdistribution busbar for carrying a distribution voltage and adistribution current, and at least one switchboard that includesprotective switchgear with contacts; a zonal power distributionsub-system including: a zonal power supply unit for supplying power toat least one electrical load, and a zonal energy store connected to theat least one switchboard of the first service distribution system forsupplying power to the zonal power supply unit; wherein the at least onepower source is regulated according to a power source foldback andstabilizing characteristic and a power source starting characteristic,and wherein the at least one electric load is regulated according to aload shedding and stabilizing characteristic; wherein the contacts ofthe protective switchgear are made to open only when the distributionvoltage and the distribution current have been reduced to acceptablelevels by the interaction of the power source foldback and stabilizingcharacteristic with one of (a) a fault that causes an excessively lowimpedance to be connected across the distribution voltage, (b) anoverriding inter-tripping command that is automatically generated withinthe power distribution system, (c) an overriding inter-tripping commandthat is manually generated within the power distribution system, and (d)an overriding inter-tripping command that is generated remotely; andwherein the contacts of the protective switchgear are made to close onlywhen the polarity of the voltage across the contacts is such that anytransient or inrush currents will be restricted by one of (a) the powersource foldback and stabilizing characteristic and the power sourcestarting sequence, and (b) the load shedding and stabilizingcharacteristic.
 2. The power distribution system of claim 1, furthercomprising a second power generation system including at least one powersource for supplying power to a second service distribution system thatincludes: at least one dc distribution busbar for carrying adistribution voltage and a distribution current, and at least oneswitchboard that includes protective switchgear with contacts; whereinthe zonal energy store of the zonal power distribution system isconnected to the at least one switchboard of the second servicedistribution system.
 3. The power distribution system of claim 2,further comprising: a first propulsion drive system; a second propulsiondrive system; a first propulsion power generation system including atleast one power source; and a second propulsion power generation systemincluding at least one power source; wherein the first propulsion drivesystem has three power supply inputs, each input being selectable, andthe first power supply input is connected to the first propulsion powergeneration system, the second power supply input is connected to thesecond propulsion power generation system, and the third power supplyinput is connected to the at least one switchboard of the first servicedistribution generation system.
 4. The power distribution system ofclaim 3, wherein the second propulsion drive system has three powersupply inputs, each being selectable, and wherein the first power supplyinput is connected to the first propulsion power generation system, thesecond power supply input is connected to the second propulsion powergeneration system, and the third power supply input is connected to theat least one switchboard of the second service power distributionsystem.
 5. The power distribution system of claim 3, wherein the firstpropulsion power generation system has first and second power supplyoutputs, each being selectable, wherein the first power supply output isconnected to the first power supply input of the first propulsion drivesystem and the second power supply output is connected to the firstpower supply input of the second propulsion drive system.
 6. The powerdistribution system of claim 5, wherein the first propulsion powergeneration system has a third power supply output that is selectable andis connected to the at least one switchboard of the first servicedistribution system.
 7. The power distribution system of claim 3,wherein the second propulsion power generation system has first andsecond power supply outputs, each being selectable, wherein the firstpower supply output is connected to the second power supply input of thefirst propulsion drive system and the second power supply output isconnected to the second power supply input of the second propulsiondrive system.
 8. The power distribution system of claim 7, wherein thesecond propulsion power generation system has a third power supplyoutput that is selectable and is connected to the at least oneswitchboard of the second service distribution system.
 9. The powerdistribution system of claim 1, wherein power is supplied to the firstservice distribution system through the at least one switchboard by oneor more of the following: the first power generation system; the zonalenergy store of the zonal power distribution sub-system; a propulsiondrive system operating in a regenerative mode; a propulsion powergeneration system; and a remote power supply system.
 10. The powerdistribution system of claim 1, wherein the at least one power source ofthe first power generation system is one or more of the following: adiesel generator; a gas turbine generator; a steam turbine generator; acombined cycle gas and steam turbine generator; a closed cycle (non-airbreathing) diesel generator; a battery; a fuel cell; a flow cell; aflywheel generator; a super-capacitor; and a superconducting magneticenergy store.
 11. The power distribution system of claim 1, wherein theat least one power source of the first power generation system isconnected to the at least one switchboard of the first servicedistribution system by a power converter.
 12. The power distributionsystem of claim 11, wherein the power converter is a pulse widthmodulated dc/dc converter.
 13. The power distribution system of claim 1,wherein the zonal energy store of the zonal power distributionsub-system is connected to the at least one switchboard of the firstservice distribution system by a power converter.
 14. The powerdistribution system of claim 13, wherein the power converter is a pulsewidth modulated dc/dc converter.
 15. The power distribution system ofclaim 14, wherein the dc/dc converter is polarized as a step-up chopperwhen power flows from the first service distribution system into thezonal energy store of the zonal power distribution sub-system, and thedc/dc converter is polarized as a step-down chopper when power flowsfrom the zonal energy store of the zonal power distribution sub-systeminto the first service distribution system.
 16. The power distributionsystem of claim 1, wherein the output voltage and output current of theat least one power source of the first power generation system areregulated such that: current flow is uni-directional; a steady stateoutput voltage is the sum of an off load bus voltage setpoint and asteady state droop component that is proportional to load current suchthat the steady state output voltage is in accordance with a steadystate load line; transient load current variations about a steady stateloading point cause the output voltage to follow a transient load linewhose gradient is less than the gradient of the steady state load line;steady state current is limited to a particular level; if load currenttransiently exceeds the steady state current limit and approaches, butdoes not exceed, a particular transient current limit level, the outputvoltage will transiently reduce with respect to the steady state loadline and will recover to the steady state load line when the steadystate current reduces below the steady state current limit; if loadcurrent continuously exceeds the steady state current limit, or exceedsthe particular transient current limit level, foldback is applied suchthat the output voltage and the output current reduce to zero accordingto a regenerative process, and output voltage and output current remainat zero until load impedance has increased beyond a particular level;and if load impedance increases beyond the particular level then loadvoltage initially partially recovers and then is ramped up to a desiredoperating point.
 17. The power distribution system of claim 16, whereinthe load voltage is ramped up to the desired operating point accordingto a time-variable ramp rate that is specified to minimize resultantvoltage transients within the power distribution system.
 18. The powerdistribution system of claim 1, wherein the distribution voltage isstabilized by a transient load line function of the power sourcefoldback and stabilizing characteristic and by a limitation of rate ofchange of load current function of the load shedding and stabilizingcharacteristic.
 19. The power distribution system of claim 1, whereinthe first power generation system includes a plurality ofparallel-connected power sources for supplying power to a first servicedistribution system, wherein the steady state current sharing of theplurality of power sources is coordinated by a steady state droopfunction of the power source foldback and stabilizing characteristic ofeach power source, and wherein the transient current sharing of theplurality of power sources is coordinated by a transient load linefunction of the power source foldback and stabilizing characteristic ofeach power source.
 20. A method of controlling a power distributionsystem comprising: a first power generation system including at leastone power source for supplying power to a first service distributionsystem that includes: at least one dc distribution busbar for carrying adistribution voltage and a distribution current, and at least oneswitchboard that includes protective switchgear with contacts; a zonalpower distribution sub-system including: a zonal power supply unit forsupplying power to at least one electrical load, and a zonal energystore connected to the at least one switchboard of the first servicedistribution system for supplying power to the zonal power supply unit;the method comprising the steps of: regulating the at least one powersource according to a power source foldback and stabilizingcharacteristic and a power source starting characteristic; andregulating the at least one electrical load according to a load sheddingand stabilizing characteristic; wherein the contacts of the protectiveswitchgear are made to open only when the distribution voltage and thedistribution current have been reduced to acceptable levels by theinteraction of the power source foldback and stabilizing characteristicwith one of (a) a fault that causes an excessively low impedance to beconnected across the distribution voltage, (b) an overridinginter-tripping command that is automatically generated within the powerdistribution system, (c) an overriding inter-tripping command that ismanually generated within the power distribution system, and (d) anoverriding inter-tripping command that is generated remotely; andwherein the contacts of the protective switchgear are made to close onlywhen the polarity of the voltage across the contacts is such that anytransient or inrush currents will be restricted by one of (a) the powersource foldback and stabilizing characteristic and the power sourcestarting sequence, and (b) the load shedding and stabilizingcharacteristic.
 21. The method of claim 20, further comprising the stepsof regulating the output voltage and output current of the at least onepower source of the first power generation system such that: currentflow is uni-directional; a steady state output voltage is the sum of anoff load bus voltage setpoint and a steady state droop component that isproportional to load current such that the steady state output voltageis in accordance with a steady state load line; transient load currentvariations about a steady state loading point cause the output voltageto follow a transient load line whose gradient is less than the gradientof the steady state load line; steady state current is limited to aparticular level; if load current transiently exceeds the steady statecurrent limit and approaches, but does not exceed, a particulartransient current limit level, the output voltage will transientlyreduce with respect to the steady state load line and will recover tothe steady state load line when the steady state current reduces belowthe steady state current limit; if load current continuously exceeds thesteady state current limit, or exceeds the particular transient currentlimit level, foldback is applied such that the output voltage and theoutput current reduce to zero according to a regenerative process, andoutput voltage and output current remain at zero until load impedancehas increased beyond a particular level; and if load impedance increasesbeyond the particular level then load voltage initially partiallyrecovers and then is ramped up to a desired operating point.
 22. Themethod of claim 21, wherein the load voltage is ramped up to the desiredoperating point according to a time-variable ramp rate that is specifiedto minimize resultant voltage transients within the power distributionsystem.
 23. The method of claim 21, wherein all the parameters of thepower source foldback and stabilizing characteristic are programmable.24. The method of claim 20, wherein the distribution voltage isstabilised by a transient load line function of the power sourcefoldback and stabilizing characteristic and by a limitation of rate ofchange of load current function of the load shedding and stabilizingcharacteristic.
 25. The method of claim 20, wherein the first powergeneration system includes a plurality of parallel-connected powersources for supplying power to a first service distribution system,wherein the steady state current sharing of the plurality of powersources is coordinated by a steady state droop function of the powersource foldback and stabilizing characteristic of each power source, andwherein the transient current sharing of the plurality of power sourcesis coordinated by a transient load line function of the power sourcefoldback and stabilizing characteristic of each power source.
 26. Themethod of claim 20, wherein the power distribution system has a powersource starting sequence where: the off load bus voltage setpoint of thepower source foldback and stabilizing characteristic is initially set tozero; the at least one power source of the first power generation systemdetects a need to start supply power by sensing one of (a) the presenceof distribution voltage resulting from the closure of the at least oneswitchboard of the first service distribution system, (b) an overridingstart command that is automatically generated within the powerdistribution system, (c) an overriding start command that is manuallygenerated within the power distribution system, and (d) an overridingstart command that is generated remotely; upon detecting a need to startsupplying power, the at least one power source is started and the offload bus voltage setpoint of the power source foldback and stabilizingcharacteristic is ramped up to a desired operating point.
 27. The methodof claim 26, wherein the load voltage is ramped up to the desiredoperating point according to a time-variable ramp rate that is specifiedaccording to the dynamic capability of the at least one power source andthe need to allow the at least one power source to progressively supplyan increasing proportion of the total load current in the powerdistribution system to minimize resultant voltage transients within thepower distribution system.
 28. The method of claim 26, wherein all theparameters of the power source starting sequence are programmable. 29.The method of claim 20, wherein the load shedding and stabilizingcharacteristic includes the step of regulating the load currentaccording to a current limit such that load current is permitted toattain any desired value but is always subject to overriding regulatorfunctions that: limit the rate of change of load current resulting fromdistribution voltage transients; and oppose any action that wouldotherwise cause load current to exceed the current limit; and where thecurrent limit: is adjustable up to and not exceeding a particular valueof current limit; is held constant when supply voltage exceeds a loadshed threshold; is progressively reduced as the supply voltage isreduced below the load shed threshold and at all levels of supplyvoltage above an absolute minimum loaded voltage; is set to zero whenthe supply voltage is less than the absolute minimum loaded voltage; isset to zero when the supply voltage increases from a value less than theabsolute minimum loaded voltage and up to a particular value; and isprogressively increased as the supply voltage is increased.
 30. Themethod of claim 29, wherein all the parameters of the load shedding andstabilizing characteristic are programmable.
 31. The method of claim 20,wherein the power distribution system has an over current protectionsequence in a situation where a low impedance fault occurs in the powerdistribution system, the over current protection sequence including thesteps of: locating the low impedance fault within the power distributionsystem; limiting the fault current and distribution voltage by applyingthe power source foldback and stabilizing characteristic; limiting theload current by applying the load shedding and stabilizingcharacteristic; detecting fault interruption; opening the contacts ofthe protective switchgear; waiting for the partial recovery of thedistribution voltage caused by the opening of the contacts of theprotective switchgear; waiting for the full recovery of the distributionvoltage caused by the application of the power source foldback andstabilizing characteristic; and waiting for the re-application of theload current caused by the application of the load shedding andstabilizing characteristic.
 32. The method of claim 20, wherein thepower distribution system has a general purpose protective or powermanagement sequence including the steps of: detecting a fault conditionor the establishment of any power management condition that requires thecontacts of the protective switchgear to be opened; generating anoverriding inter-tripping command; limiting the distribution voltage byapplying the power source foldback and stabilizing characteristic;limiting the load current by applying the load shedding and stabilizingcharacteristic; detecting a load current interruption; opening thecontacts of the protective switchgear; waiting for the partial recoveryof the distribution voltage caused by the opening of the contacts of theprotective switchgear; waiting for the full recovery of the distributionvoltage caused by the application of the power source foldback andstabilizing characteristic; and waiting for the re-application of theload current caused by the application of the load shedding andstabilizing characteristic.
 33. The method of claim 20, wherein thezonal energy store of the zonal power distribution sub-system receivespower from, or supplies power to, the first service distribution system,the power being regulated for the purpose of: re-charging the zonalenergy store; supplying power to the zonal power supply of the zonalpower distribution sub-system; supplying power to the first servicedistribution system; providing a bulk energy store; supplying powercontinuously for any purpose; supplying power transiently to assist instabilizing the distribution voltage; supplying power transiently tosupport other power sources that have a poor transient response;providing isolation between the zonal energy store and the first servicedistribution system to allow the zonal power supply to operateindependently when the first service distribution system is subject to afailure; or allowing the first service distribution system to operateindependently of the zonal energy store when the zonal energy store orthe zonal power supply is subject to a failure.